Most of the time, we think of space-faring rockets as being launched from the ground. As demand for satellite launch services rapidly increases year-over-year, interest in air launching rockets is returning to a growing market of lighter-weight payloads. And those might want a mothership.

Air launches are fairly rare, as space launch systems go. Virtually every spacecraft you’ve ever seen has been mounted on top of a big pointy rocket, sitting vertically atop the ground. But that’s sort of odd, as air launches have a huge advantage over ground launches.

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An air launch, where the rocket is dropped from an air-breathing mothership that’s borne aloft by wings and aerodynamic principles instead of pure thrust, tends to be vastly cheaper than a ground launch.

By using the lowest and thickest part of the atmosphere to lift the wings of the mothership, instead of forcing the sheer power of the rocket to push through it itself, you can save on a lot of fuel (and thus, money). And by cutting out the first and hardest 40,000 feet of the atmosphere using an air-breathing-mothership-and-drop approach, you can cut potentially millions from you budget. It’s great. On paper.

The problem is that motherships can only get so big. Some of the biggest motherships, like the B-52 bomber-based “Balls 8” aircraft, used by NASA, or the Lockheed L-1011 airliner-based Orbital Stargazer used by private space company Orbital ATK, which could only drop a maximum of 51,000 pounds. And those were huge planes.

If you had a payload and rocket that weighed more than what planes like that could feasibly lift to up 40,000 feet and then drop, then you needed to start your rocket off from the ground, which would require even more rocket fuel. Which would need even more fuel to lift all that fuel. Which would cost even more money.

The costs increase exponentially, to the point where the largest rocket ever built, the Saturn V, could cost $3.2 billion per launch in present day dollars.

But if you’ve only got something small that needs to go into space, it’s hard to beat the mothership approach. Air launch systems have other advantages too, such as the ability to launch from practically any runway, thereby avoiding weather that could delay a ground launch, as well as launching the rocket in any direction. Perhaps most importantly, the mothership itself is reusable, meaning a big portion of the launch system’s architecture doesn’t have to be built from scratch every time. For smaller payloads, motherships represent the future of space travel.

So, if you’ve got a compact, lightweight payload (like a small Earth observing satellite from Google subsidiary SkyBox Imaging, or an Arkyd asteroid-locating satellite from Planetary Resources) that needs to hitch a ride to low earth orbit, these are pretty much the options you’re working with right now for all your air-launching needs.

White Knight/WhiteKnightTwo

Billionaire Richard Branson’s Virgin Galactic has been promising to send tourists into space for years, and the architecture they’ve helped to develop, is another example of how payloads can be launched from the air. The company has been gearing up for routine private spaceflights for over a decade, and over 800 people (including Steven Hawking, Leonardo DiCaprio and Ashton Kutcher) have placed deposits to be among the spaceline’s first passengers.

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Both the White Knight and WhiteKnightTwo motherships were designed by Scaled Composites, with their designs based on the firm’s successful Proteus concept. The White Knight (officially renamed White Knight One to more easily distinguish between the two carrier aircraft) is now retired and on display at fellow billionaire Paul Allen’s Flying Heritage Collection in Everett, Washington.

WhiteKnightTwo carries SpaceShipTwo spaceplanes to around 50,000 feet before their separation. And even though SpaceShipTwo is technically a suborbital vehicle, WhiteKnightTwo was also designed to carry Virgin Galactic’s LauncherOne system for sending commercial cubesat and microsatellite payloads (up to 440 pounds) into orbit, although Virgin recently announced that LauncherOne would be carried aloft by its own dedicated carrier aircraft, apparently due to heavier than expected demand. However, it is possible that WhiteKnightTwo could still be used as a mothership for a future air launch rocket project.

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At one point, WhiteKnightTwo was even being considered for use as an aerial firefighting platform. Scaled Composites designed the WhiteKnightTwo to lift 37,000 pounds to 50,000 feet, so carrying a specially-designed tank and thousands of gallons of water or retardant shouldn’t pose any issues.

Virgin Galactic initially planned its first suborbital flights with paying customers in 2015. However, the loss of SpaceShipTwo during a test flight last year has added delays to Virgin’s timeline. Meanwhile, the LauncherOne system’s first flight is slated to launch next year, with costs estimated at under $10 million.

The commercial viability of WhiteKnightTwo remains to be seen at this point, although its potential multipurpose capabilities are a good sign that the investment in this unique mothership aircraft will eventually pay off.

Stratolaunch

Although it isn’t yet operational, a new mothership is on the horizon, and it is going to be enormous. The Stratolaunch system, which is currently in development, is a collaboration between billionaire Paul Allen, aerospace legend Burt Rutan and former NASA Administrator Mike Griffin. The company is constructing a massive new carrier aircraft to hoist heavier payloads into orbit on air launched rockets.

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In 2012, it was announced that Orbital was providing the launch vehicle, to be called Pegasus II, for the Stratolaunch system. However, the Pegasus II was eventually deemed unable to meet Stratolaunch’s economic criteria, and by May 2015 Stratolaunch pivoted to seek new commercial partners for their launch vehicle. We don’t yet know how many pounds the launch vehicle will be able to deliver to orbit, but it is reasonable to expect that compared to other air launch systems, the capacity will be comparatively huge.

When Stratolaunch comes online, the carrier aircraft will have the longest wingspan (385 feet) of any airplane to ever fly. The first flight is slated for 2016, and will likely take place out of the Stratolaunch facilities in Mojave, California. When paired with its fueled rocket, the massive mothership will weigh a staggering 1.2 million pounds. To put that in perspective, that’s roughly the same as the Airbus A380’s maximum takeoff weight.

ALASA and SALVO

DARPA’s ongoing ALASA (Airborne Launch Assist Space Access) program is also seeking to create air launch solutions for placing 100-pound satellites into low Earth orbit. ALASA’s big aim is to deliver small payloads to orbit at a cost of less than $1 million per launch.

The ALASA rocket will be 24 feet long and will be carried aloft by an unmodified F-15E Strike Eagle carrier aircraft. Once the F-15E reaches 39,000 feet, ALASA will separate and proceed to orbit. ALASA’s first flight is scheduled for later in 2015.

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However, a smaller version of ALASA called SALVO (Small Air Launch Vehicle to Orbit) has reportedly already been tested using the same F-15E carrier aircraft. SALVO has been described as an “icebreaker” for ALASA, meaning that the lessons DARPA learns from the smaller SALVO launch vehicle can be applied directly to the ALASA program.

Other Air Launch Systems

Another company joining the air launch game is Generation Orbit Launch Services. They are preparing to offer air launch services for small satellites using a modified Gulfstream G-III business jetcarrier aircraft, and flew a successful test flight last year.

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Finally, the military, the CIA, or the National Reconnaissance Office (NRO) are speculated to have operated one or more highly classified, high speed mothership(s) out of the shadowy base at Groom Lake, Nevada, better known as Area 51. Such a carrier aircraft could carry a smaller and even faster spaceplane to a high altitude before separation. One of these dark motherships may have resembled, or could have evolved from, the North American XB-70 Valkyrie bomber. Classified programs with names like Blackstar, Brilliant Buzzard, Copper Coast and Science Dawn have all supposedly referenced similar concepts.

Analysts have speculated that one potential use for the massive new hangar under construction at the south end of the runway at Area 51 is to shelter some sort of mothership-parasite configuration. While we have no confirmation that any such air launched military space program existed or achieved operational capability, the capabilities that such a system could offer would certainly be attractive to any military strategist.

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Given that satellites are vulnerable to anti-satellite missiles and other space-based threats, the ability to quickly and cheaply replace them in a time of crisis would be a very good military strategy. Given the military’s reliance on satellites for navigation, communication, weapons guidance and a host of other functions, it is plausible that an air launch system could have been designed to rapidly insert critical replacement satellites following the loss of another spacecraft.

Onward and Upward

Minimal launch infrastructure, mobility, reusability and lower cost per launch are all very compelling reasons why air launching rockets seems like a really smart way to develop our infrastructure in space. As demand for space access continues to grow, especially for lightweight payloads, it is no surprise that several air launch systems are likely to reach operational maturity in the coming years.

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However, just like launching from the ground, air launched access to space is an incredibly risky business and certainly not without peril. Moreover, developing these motherships are incredibly expensive and may ultimately prove to be a money-losing endeavor. But as space becomes increasingly commercialized, we may soon be witnessing an air launched renaissance. And for those of us who love both airplanes and rockets, what could be better than a combination of the two?